1. Field of the Invention
The present invention relates to a semiconductor laser module and a laser wavelength control apparatus used for optical communication.
2. Description of the Related Art
FIG. 1 shows a conventional semiconductor laser module which can be used for optical communication.
Referring to FIG. 1, reference numeral 11 denotes a semiconductor laser. The oscillation wavelength (oscillation frequency) of the semiconductor laser 11 can be controlled by changing its injection current or temperature. Light S.sub.1 emitted from the left side of the semiconductor laser 11 in FIG. 1 is focused on an optical fiber 14 for optical transmission through optical lenses 12 and 13. Light S.sub.2 emitted from the right side of the semiconductor laser 11 in FIG. 1 is collimated by an optical lens 15 and is split by a beam splitter 16 in two directions. One split light component S.sub.3 is focused on a first photodetector (e.g., a photodiode) 18 through a optical lens 17. The other light component S.sub.4 undergoes a change in intensity through a Fabry-Perot resonator 19 and is focused on a second photodetector (e.g., photodiode) 21 through an optical lens 20.
The Fabry-Perot resonator 19 is designed such that a pair of reflecting mirrors 191 and 192 each consisting of a dielectric multilayer film are set parallel and oppose each other at a distance L. The Fabry-Perot resonator 19 has a characteristic that a light intensity is repeatedly changed at a period of a free spectral interval C/2nL (C: light velocity; n: refractive index in the Fabry-Perot resonator) with respect to the frequency of incident light, as shown in FIG. 2. For this reason, light which is incident on the Fabry-Perot resonator 19 undergoes a change in intensity in accordance with its frequency, and a detection output from the second photodetector 21 undergoes a change in level due to the change in intensity. Therefore, the oscillation wavelength of the semiconductor laser 11 can be obtained by measuring a ratio of an output from the first photodetector 18, which receives light free from an intensity change, to an output from the second photodetector 21, which receives light which is changed in intensity.
In the conventional apparatus, therefore, as basically shown in FIG. 3, both outputs from the first and second photodetectors 18 and 21 are input to a feedback control unit 22, and an oscillation wavelength is obtained by the control unit 22 on the basis of the level difference between the outputs. The temperature or injection current of the semiconductor laser 11 is changed in accordance with the obtained oscillation wavelength, thereby controlling the oscillation wavelength of the semiconductor laser 11 to be a desired oscillation wavelength. FIG. 4 shows a relationship between the oscillation wavelength of the semiconductor laser 11 and a detection level difference from the feedback control unit 22, and also shows the wavelength capture range of the control unit 22.
The conventional Fabry-Perot resonator 19 used in the above-described semiconductor laser module must be designed under the following conditions and limitations. In the first place, the two reflecting mirrors 191 and 192 must be arranged with a parallelism on the order of seconds. In the second place, the two reflecting mirrors 191 and 192 must be arranged at the interval L on the order of submicrons. In the third place, the ambient temperatures of the two reflecting mirrors 191 and 192 must be controlled with a precision of 0.1.degree. C. or less because the two reflecting mirrors 191 and 192 and their holder (not shown) expand or contract depending on changes in temperature and humidity so as to change the distance L. In the last place, the positions of the two reflecting mirrors 191 and 192 tend to shift from each other due to an external impact. Because of these limitations, the Fabry-Perot resonator 19 is difficult to manufacture. In addition, the Fabry-Perot resonator 19 tends to exhibit variations in characteristics and is susceptible to variation due to external factors. Therefore, stable control of the wavelength of the semiconductor laser is difficult.
In the above-described means for controlling the wavelength of a semiconductor laser, a set wavelength is not located at the center of the wavelength capture range, as shown in FIG. 4. Since the wavelength capture range cannot be effectively used for stable feedback control, such a means is difficult to operate. Especially, if the fineness of the Fabry-Perot resonator 19 is increased to improve its sensitivity, this tendency becomes more conspicuous. Therefore, the sensitivity is difficult to improve. In addition, since control by this means is performed in a DC manner, its operation is susceptible to drifts. That is, the set wavelength precision is affected not only by changes in sensitivity of the photodetectors 18 and 21, changes in sensitivity of an amplifier, arranged in the feedback control unit 22, for amplifying an input signal, and a O-point drift but also by changes in light amount due to dust and the like in an optical path. For this reason, it is very difficult to stabilize a set wavelength over a long period of time.